1. Field of the Invention
The present invention relates to a process for the oligomerization of olefins, in which an olefin is brought into contact with a catalyst system comprising at least one transition metal complex with a polydentate complexing ligand and an alkylaluminoxane.
2. Description of the Background
Olefin oligomers having up to 30 carbon atoms have great economic importance as comonomers for plastics or as precursors for oxo alcohols which are in turn constituents of surfactants and plasticizers for plastics. Processes for the oligomerization of lower olefins produced in, for example, steam crackers are thus of central importance in the production of products used in daily life.
WO 00/58319 describes a process for preparing oligomers of olefins using an oligomerization catalyst which is obtainable from a chromium compound and a 1,3,5-triazacyclohexane and an activating additive such as an alkylaluminoxane.
EP-A-0 537 609 describes the oligomerization of ethene in the presence of a catalyst comprising a chromium complex with a polydentate complexing ligand and an alkylaluminoxane.
Although the mechanism of olefin oligomerization over transition metal complexes has not been fully elucidated, it is assumed that the “activation” of the transition metal complex by the alkylaluminoxane involves a ligand exchange reaction between an abstractable ligand of the complex and the alkyl group of the alkylaluminoxane. This forms a catalytically active species which adds on olefin molecules in a stepwise fashion. The olefin molecules react in the coordination sphere of the transition metal complex to form an oligomer. Liberation of the oligomer regenerates the catalytically active species. However, even with substantial exclusion of impurities, the catalytic activity of the catalyst system becomes exhausted after some time. In general, the catalyst system is then discarded.
The alkylaluminoxane is usually used in a large excess based on the transition metal complex in order to achieve satisfactory activation. The associated high consumption of alkylaluminoxane therefore represents a considerable cost factor in such olefin oligomerization processes.